Download Chapter 20 The Precambrian Record

170
Chapter
20
The Precambrian Record
GUIDED STUDY
The text chapter should be studied one section at a time.
Before you read, preview each section by skimming it,
noting headings and boldface items. Then read the
appropriate section objectives from the following
outline. Keep these objectives in mind and, as you read
the chapter section, search for the information that will
enable you to meet each objective. Once you have
finished a section, write out answers for its objectives.
Archean Geology (pp. 404-409)
4. Describe the vertical succession of rocks that
characterize a greenstone belt. What evidence
suggests that such sequences formed underwater?
The Divisions of Precambrian Time (p. 400)
1. What characteristics of Precambrian rocks caused
geologists in the eighteenth and nineteenth centuries
to consider them as Earth’s oldest rocks?
The Hadean Eon (pp. 401-404)
5. Describe four processes by which isolated patches
of continental crust could be assembled into larger
cratons.
6. Archean sedimentary rocks are mostly graywackes,
cherts and volcanic sediments. Why were shallow
marine and terrestrial sedimentary rocks rarely
formed at this time?
2. What information about the Moon was provided by
the lunar samples collected in the 1960s and 1970s?
Proterozoic Geology (pp. 409-413)
7. Describe how changes in tectonic activity during the
Proterozoic Eon provided new environments for
deposition of shallow marine sediments.
3. What does the occurrence of the ultramafic rock
komatiite of Archean age suggest about the
composition of the Earth’s first crust?
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171
8.
9.
Describe the three-part sequence of events involved
in the “modern” style of plate tectonics, termed a
Wilson cycle.
What evidence suggests that Laurentia experienced
extensive tectonic rifting during the Proterozoic
Eon?
Precambrian Life (pp. 416-419)
14. What environment is now thought to be the most
likely site for the origin of life?
15. What evidence suggests that eukaryotic cells
originated from more primitive prokaryotic forms?
CHAPTER REVIEW
The Precambrian Atmosphere and Climate
(pp. 413-416)
10 What process was responsible for generation of the
Earth’s first permanent atmosphere?
11. What two sources provided water for the Earth’s
oceans?
When you have finished reading the chapter, work
through the material that follows to review it. Complete
the sentences. As you proceed, evaluate your
performance for each section by consulting the answers
on pages 177-178. Do not continue with the next section
until you understand each answer. If you need to, review
or reread the appropriate section in the textbook before
continuing.
Introduction (pp. 399-400)
1.
________________ is the scientific method used
to determine the age of Earth’s oldest rocks.
2.
An age of 4.6 billion years comes from
measurements made of ________________.
3.
12. What two processes were responsible for the
dramatic increase in oxygen levels during the
Proterozoic Eon?
Tectonic processes destroy the oceanic crust, so
Earth’s oldest crustal rocks occur on the ________.
4.
_______________ is the region that can claim the
Earth’s oldest exposures of surface rocks.
5.
Earth’s oldest mineral grains are _________ found
in Australia that date between 4.0-4.4 billion years.
13. What is a possible mechanism to explain the late
Proterozoic glaciation that apparently covered even
tropical land masses?
The Divisions of Precambrian Time (p. 400)
6. The rocks of presumed great age found in deep
gorges and mountain belt cores were termed
_____________.
7. Lack of ___________ in these metamorphosed and
deformed Precambrian rocks made their correlation
impossible.
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172
8. Divisions used by geologists that study Precambrian
21. The rock associations that contain interlayered
rocks are based upon changes in _________
volcanic and sedimentary rocks that are
_______ and the ______________ of the rocks in
metamorphosed and folded into broad troughs are
each interval.
called_______ ______.
9. The most reliable method for correlation of
Precambrian rocks is ___________ ________.
22. The oldest volcanic rocks at the base of a
greeenstone belt sequence are ______________.
23. Sedimentary formations interlayered with the
The Hadean Eon (pp. 401-404)
volcanic rocks contain dark shales and turbidites
10. The earliest events of the Hadean involved the
formed in ________ environments.
____________ and_______________ of the Earth.
11. The Moon’s lower density indicates that it has a
much smaller metallic________ than that of Earth.
12. The Moon is hypothesized to have been acquired
by the Earth’s gravitational field according to the
old idea of _______________ ____________.
13. The chemical differences between Earth and the
Moon do not support the hypothesis of ______
___________.
14. Because it explains the composition and orbit of the
Moon, the _______________hypothesis is
24. The tectonic location of greenstone belts is probably
in forearc or backarc basins near _________ _____.
25. Small patches of continental crust were joined
together to form larger _______ by a variety of
tectonic processes.
26. Larger continental blocks were unable to form
because the elevated temperature of the crust
produced widespread _______________.
27. Archean sedimentary rocks do not include
sandstone or limestone because the continental
blocks lacked broad, shallow_____________.
currently considered the most likely explanation for
the origin of the moon.
15. The entire ________ of the Earth may have melted,
due to heat from accretion, impacts and radioactive
decay from the interior.
16. The composition of the Earth’s first crust was
__________ , as indicated by the presence of very
old Archean lavas called komatiites.
17. Intermediate to felsic magmas that formed the
Earth’s earliest continental crust may have formed
by the process of ________ _____________.
18. Zircons 4 –4.4 billion years old from Australia
indicate the oldest possible ___________ _______.
Proterozoic Geology (pp. 409-413)
28. Proterozoic rocks are more commonly exposed and
less ___________ than Archean rocks.
29. Slower Proterozoic mantle convection produced
larger continents with sediment deposition on broad
shelves at the _____________ _______.
30. Proterozoic marine sediments were also deposited
in shallow seaways called ________ _______,
which covered large parts of the cratons.
31. The accumulation of coarse detrital sediments and
volcanic rocks in fault-bounded basins indicates the
process of _______________ ____________, the
initial stage of a Wilson cycle.
Archean Geology (pp. 404-409)
19. Archean rocks are exposed in the ________areas of
the continents.
32. The accumulation of marine sediments along
passive margins indicates _____________ of rifted
continental fragments.
20. The rock association that represents the original
Archean continental crust is called_______ ______.
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173
33. Deformation of passive margins rocks by
subduction and continental collision represents
__________ of ocean basins.
34. The elongated belt of deformed rocks that forms at
the plate edge from the tectonic processes of a
Wilson cycle is called a(n) __________.
35. The oldest known Wilson cycle is preserved in a
belt of folded, metamorphosed, and intruded rocks
in Canada called the __________ __________.
36. The assembly of all major shield provinces of
_____________.
45. Although cooling of the Earth’s crust is probably
the source of most water found in the oceans,
impact of _________ may have provided additional
volumes of water.
46. Splitting of water molecules by ultraviolet radiation
in the atmosphere releases free oxygen by the
process of ____________ ________________.
47. Oxygen produced by the process of
_________________ dissolved in the ocean waters
Proterozoic age in North America formed the
and combined with other elements to form oxide
continent of _______________.
minerals.
37. The southwestern portion of North America is
48. Significant levels of dissolved oxygen in the early
called the _______________ province. It represents
Proterozoic seas are indicated by the abundance of
a series of volcanic island arcs that were accreted to
_______ ______ ____________, which form when
the southern margin of Laurentia.
dissolved iron combines with oxygen on the ancient
38. The youngest addition to the North American
craton during the Proterozoic occurred during the
sea floor.
49. Precambrian ice ages seem to have resulted from
___________ ________, where North America
plunging levels of atmospheric carbon dioxide,
apparently collided with South America.
which may have been caused by excessive
39. Crustal stretching during the middle Proterozoic
formation of ______________ rocks.
formed the ___________ _______, exposed at the
surface only in the Great Lakes region.
40. The late Proterozoic supercontinent of __________
was composed of Africa, South America, Europe,
North America, Australia, Antarctica, and India.
41. Basaltic lavas extruded in the Great Lakes region
Precambrian Life (pp. 416-419)
50. Living things are composed of organic molecules
with the ability to metabolize and ___________.
51. Because of igneous activity and the constant
bombardment from space, the environment most
during the middle Proterozoic were rich in the
likely to have fostered life was probably the deep
metal __________.
ocean near _______________ _______.
The Precambrian Atmosphere and Climate
52. The earliest fossils from 3.5 billion year-old cherts
(pp. 413-416)
of Australia resemble modern prokaryotes called
42. The Earth’s first atmosphere of nebular gases was
__________________.
probably swept away by the blast of particles
generated from the sun known as the _____ _____.
43. The release of large volumes of gases and water
vapor to the surface from volcanic activity is called
_________ ____________.
44. The composition of Earth’s early atmosphere can
be estimated from the gases exhaled from modern
Copyright © Houghton Mifflin Company. All rights reserved.
53. _______________ are the most important fossil
type of the Archean and Proterozoic eons.
54. Coiled ribbons of multicellular _______ represent
the oldest known eukaryote fossils.
55. Eukaryotes may have formed by the union of
ingested prokaryotic components in a process called
_________________.
174
56. Multicellular organisms in which groups of cells
perform specialized tasks are called __________.
57. Disk or frond-shaped fossil impressions of late
Proterozoic age found represent the __________
________.
58. Late Proterozoic locomotion is indicated by ______
__________ of crawling animals.
59. The first appearance of hard skeletal fossils is
represented by small _________ _______.
60. Diversity of metazoans in the late Proterozoic was
c. the dual formation hypothesis
d. the planetary capture idea
6. The first crust of the Earth was destroyed by:
a. plate tectonics.
b. meteorite impacts.
c. melting of the Earth’s mantle.
d. All of the above.
7. The hottest lavas that ever erupted on the Earth’s
surface formed ultramafic volcanic rocks called:
a. basalts.
b. komatiites.
c. andesites.
d. rhyolites.
due to a warming of the oceans and an increase in
the levels of ___________.
PRACTICE TESTS
After you thoroughly understand the correct answers of
the Chapter Review, answer the following questions and
check them with the answers on pages 178-179. If your
answer is incorrect, consult the appropriate pages of the
text.
Multiple Choice Questions
Circle your answers to the following questions.
1. The oldest rocks on Earth would be:
a. the rocks of the ocean basins.
b. the rocks of the folded mountain belts.
c. the rocks of the shield areas.
d. fragments of meteorites that landed on Earth.
2. Earth’s oldest known crustal rocks are found in:
a. Canada.
c. Antarctica.
b. South Africa.
d. China.
3. Primary rock formations were considered to be:
a. uncorrelatable.
b. devoid of fossils
c. deformed and metamorphosed.
d. All of the above.
4. The eons of the Precambrian combined account for:
a. 27% of all geologic time.
b. 42% of all geologic time.
c. 87% of all geologic time.
d. 97% of all geologic time.
5. Which hypothesis concerning the origin of the
Moon best explains its differences in
composition from that of Earth?
a. the nebular hypothesis
b. the impactor hypothesis
8. The oldest rock formation that represents
continental crust consists of a rock called:
a. sandstone.
b. komatiite.
c. tonalite.
d. greenstone.
9. Zircon mineral grains from Australia that are dated
at 4.0-4.4 billion years old indicate a felsic
continental crust that is:
a. over 4 billion years old.
b. about 3.5 billion years old.
c. about 3 billion years old.
d. about 2.5 billion years old.
10. The most common Archean continental crust rocks
are found:
a. in greenstone belts.
b. in granulite gniess associations.
c. volcanic island arcs.
d. continental shelves.
11. Rocks of the greenstone belt associations indicate:
a. deposition on land.
b. deposition along continental margins.
c. deposition in deep water of backarc basins.
d. deposition in oceanic trenches
12. Small patches of continental crust were enlarged by:
a. intrusions of magma along plate edges.
b. accretion of volcanic arcs at subduction zones.
c. welding of sediment wedges at plate margins.
d. all of the above.
13. Plate tectonics in the Archean Eon was
characterized by:
a. rapid plate movement and frequent collisions.
b. rifting of large continents.
c. drifting of large continents.
d. collision of large continents.
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175
14. The small, narrow continental blocks of the
Archean were ideal depositional sites for:
a. sandstones.
c. graywackes.
b. limestones.
d. conglomerates.
15. Formation of broad shallow shelves and epeiric seas
was afforded by:
a. an increase in mantle plume activity.
b. a decrease in the rate of mantle convection.
c. an increase in rifting of continental blocks.
d. an increase in collision of continental blocks.
16. The Wilson cycle illustrates:
a. the “old” Archean tectonic style of plate motion.
b. the “old” Archean style of mantle convection.
c. the “modern” style of plate motion.
d. the “modern” style of mantle convection.
17. The first stage of a Wilson cycle involves:
a. rifting of continents to form ocean basins.
b. drifting of rifted continental fragments.
c. closure of ocean basins by subduction and
continental collision.
d. accretion of volcanic arcs to plate margins.
18. The deposition of quartz-rich sandstones and
carbonates of the Odjick formation in the
Wopmay orogen indicates which stage of a
Wilson cycle?
a. rifting of continents to form ocean basins.
b. drifting of rifted continental fragments.
c. closure of ocean basins by subduction and
continental collision.
d. accretion of volcanic arcs to plate margins.
19. Thrusting of the Coronation Supergroup formations
eastward over the Slave craton illustrates which
stage of a Wilson cycle?
a. rifting of continents to form ocean basins.
b. drifting of rifted continental fragments.
c. closure of ocean basins by subduction and
continental collision.
d. accretion of volcanic arcs to plate margins.
20. The continent of Laurentia was assembled during
the early Proterozoic by the process of:
a. rifting.
b. drifting.
c. subduction and continental collision.
d. accretion of volcanic arcs to plate margins.
21. The total, subsurface extent of the Midcontinent rift
can be determined by which rock type?
a. basaltic lavas.
b. conglomerates.
c. sandstones.
d. graywackes.
Copyright © Houghton Mifflin Company. All rights reserved.
22. The assembly of Rodinia was accomplished in part
by the:
a. Midcontinent rift.
b. Trans-Hudson orogen.
c. Mazatal orogen.
d. Grenville orogeny.
23. To survive, the earliest atmosphere would have
required:
a. intense surface igneous activity.
b. formation of the Earth’s magnetic field.
c. bombardment by comets.
d. the presence of plants performing
photosynthesis.
24. Salinity in the oceans developed from:
a. magmas with a high salt content.
b. comets with a high salt water content
c. rainwater with a high salt content.
d. chemical weathering of the continents by highly
acidic surface waters.
25. Elevated oxygen levels on land in the later
Proterozoic are best indicated by the formation
of:
a. banded iron formations.
b. glacial deposits.
c. redbeds.
d. salt deposits.
26. Solar energy during the Precambrian may have been
only 70% of today’s levels. How did the Earth
stay warm?
a. limestone production warmed the Earth
b. elevated carbon dioxide levels warmed the
Earth
c. the Earth was closer to the Sun
d. the continents were located on the equator
27. Proteins, the building blocks of cells, are composed
of:
a. the nucleic acid DNA
b. the nucleic acid RNA.
c. protenoids.
d. amino acids.
28. Stromatolites are sedimentary structures formed by:
a. sticky cyanobacteria mats that bind sediments.
b. shallow waves
c. swiftly-moving streams.
d. impressions of the Ediacara metazoans.
29. The best evidence for endosymbiosis lies with the:
a. organelles that contain their own proteins.
b. organelles that contain their own amino acids.
c. organelles that contain their own DNA.
d. organelles that contain their own single cell wall.
176
30. The Ediacaran fauna represents:
a. unicellular eukaryotes.
b. multicellular eukaryotes
c. unicellular prokaryotes.
d. multicellular prokaryotes.
True or False Items
CHALLENGE TEST
Answer these questions the day before an exam as a
final check on your understanding of the chapter’s terms
and concepts. Check your responses with the answers on
pages 179-180. If your answer is incorrect, consult the
appropriate pages of the text (in parentheses following
the correct answer).
Write true or false on the line in front of each statement.
Completion
_____ 1. Fossils provide us with accurate
determinations of the age of the Earth.
_____ 2. The oldest Earth materials are 4.0-4.4
billion-year old zircon grains from Australia.
_____ 3. Precambrian rocks encompass 87% of all
geologic time.
_____ 4. The dual formation hypothesis best explains
the origin of the Moon.
_____ 5. Formation of Earth’s crust followed the
formation of the Moon.
_____ 6. The first continents were composed of
ultramafic and mafic rocks.
_____ 7. Archean continental crust is represented by
rocks contained in greenstone belts.
_____ 8. Archean continents remained small because
of frequent rifting.
_____ 9. Large Proterozoic continents developed
because of slower rates of mantle convection.
_____ 10. A Wilson cycle describes the tectonic
processes responsible for growth of small
Archean continents.
_____ 11. Laurentia was formed by a series of
continental rifting events.
_____ 12. Earth’s present atmosphere is characterized
by the presence of the ozone layer.
_____ 13. Higher levels of carbon dioxide in the
Precambrian atmosphere kept temperatures
warm because of a greenhouse effect.
_____ 14. Prokaryotes are primitive, nonnucleated
cells.
_____ 15. Ediacaran metazoans were capable of
making hard skeletons.
Essay Questions
Write a brief essay on a separate sheet of paper
answering each of the following questions.
1. List and briefly discuss several hypotheses that
explain the Moon’s formation.
2. What evidence is there to suggest that the Earth’s
early atmosphere was compositionally very
different from that of today? What changes took
place to cause the modern atmosphere to form?
3. What could have caused the extreme glacial ages of
the late Proterozoic ?
Fill in the correct answers.
1. The three subdivisions of the Precambrian are the:
___________________, ___________________,
and ____________________ eons.
2.
The three ideas used to explain the origin of the
Moon are the _____________ ____________,
________ ______________, and __________
hypotheses.
3. The composition of Earth’s first crust is believed to
be ____________.
4. The felsic continental crust was probably formed by
the process of________ ___________.
5. The greenstone belts probably formed under water
because they contain _______ __________.
6. The Archean continents remain small because of
continual _______________.
7. Archean sedimentary rocks were deposited maily in
_______________ environments, because the
continents were small, narrow, and steep-sided.
8. Proterozoic tectonics facilitated the growth of larger
continents, and the appearance of broad, shallow
shelves that favored deposition of
_______________ (rock type).
9. The last phase of a Wilson cycle produces folding
and metamorphism, which are produced by
subduction and _____________ ____________.
10. An________ is an elongated belt of deformed rocks
that forms at the edge of a plate through tectonic
processes.
11. In North America, the assembly of Rodinia
involved the __________ __________, in which
Copyright © Houghton Mifflin Company. All rights reserved.
177
western South America and eastern North America
collided.
12. Proterozoic glacial episodes are indicated by the
presence of varved sediments and _________.
13. The most popular theory for the origin of
eukaryotes is called _____________________.
Multiple-Choice Questions
Circle the correct answer.
_____ 2. The age of Precambrian rocks can be
determined from the severity of their
deformation.
_____ 3. The overall density of the Moon is much less
than that of Earth.
_____ 4. Komatiite lavas indicate a much hotter
Archean Earth.
_____ 5. The “modern” style of plate tectonics began in
the Proterozoic with a slower rate of mantle
convection.
_____ 6. Stromatolites were the most advanced fossil
forms throughout the entire Precambrian
interval.
1. Earth’s oldest rocks are technically:
a. meteorites.
c. tonalites.
b. granulites.
d. sandstones.
ANSWERS
2. The oldest interval of the Earth is the:
a. Phanerozoic Eon.
c. Hadean Eon.
b. Proterozoic Eon.
d. Archean Eon.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
3. The Moon was probably derived by impact from the
Earth’s:
a. core
c. mantle.
b. crust.
d. asthenosphere.
4. The Archean rocks of the Canadian Shield have
been assigned to distinct crustal provinces based on:
a. different folding types. c. different ages.
b. different compositions. d. all of the above.
5. Archean ultramafic lavas that indicate a hotter Earth
surface are called:
a. tonalite.
c. quratzite.
b. komatiite.
d. granulite.
6. Sedimentary rocks characteristic of Archean
deposition are:
a. sandstones.
c. graywackes.
b. conglomerates.
d. limestones.
7. The second phase of a Wilson cycle would be:
a. rifting continents.
c. colliding continents.
b. drifting continents.
d. shifting continents.
8. The most advance Proterozoic organisms were:
a. unicellular prokaryotes.
b. multicellular prokaryotes.
c. unicellular eukaryotes.
d. multicelular eukaryotes.
True or False Items
Write true or false on the line in front of each statement.
_____ 1. Earth’s oldest crust is exposed in rocks of
Greenland.
Copyright © Houghton Mifflin Company. All rights reserved.
CHAPTER REVIEW
radioactivity
meteorites
continents
Canada
zircons
primary
fossils
tectonic activity; composition
radiometric dating
accretion; differentiation
core
planetary capture
dual formation
impactor
mantle
ultramafic
partial melting
continental crust
shield
granulite gniess
greenstone belts
komatiites
deep-water
subduction zones
cratons
rifting
shelves
deformed
continental margins
epeiric seas
continental rifting
drifting
closure
orogen
Wopmay orogen
Laurentia
Mazatal
Grenville orogeny
Midcontinent rift
Rodinia
copper
178
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
solar wind
volcanic outgassing
volcanoes
comets
photochemical dissociation
photosynthesis
banded iron formations
limestone
reproduce
hydrothermal vents
cyanobacteria
stromatolites
algae
endosymbiosis
metazoans
Ediacaran fauna
trace fossils
calcite tubes
oxygen
PRACTICE TESTS
Multiple-Choice Questions
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
d
a
d
c
b
d
b
c
a
b
c
d
a
c
b
c
a
b
c
c
a
d
b
d
c
b
d
a
c
b
True or False Items
1. False. Radiometric dating of meteorites provides
accurate determinations of the age of the Earth.
2. True.
3. True.
4. False. The impactor hypothesis best explains the
origin of the Moon.
5. True.
6. False. The first oceanic crust was composed of
ultramafic rocks.
7. False. Archean continental crust is represented by
rocks of the granulite gniess association.
8. True.
9. True.
10. False. A Wilson cycle describes the modern
sequence of plate tectonic processes.
11. False. Laurentia was built by a series of continental
collisions.
12. True.
13. True.
14. True.
15. False. Organisms that lived at the same time as the
Ediacaran metazoans made hard, calcite skeletons.
Essay Questions
1. Several hypotheses have been proposed to explain
the origin of the Moon.
a. The oldest idea is that of planetary capture,
which suggests that the Moon was a planetary body
that formed elsewhere and was later captured by the
Earth’s gravitational field. The likelihood of such an
event is very small, considering the small size of the
Moon and the approach necessary to permit capture
without causing a catastrophic collision.
b. The dual accretion hypothesis proposes
simultaneous formation of the Earth and the Moon
with the orbital dynamics such that the smaller
Moon orbits the more massive Earth. The
compositional differences between these two bodies
does not support such an origin, as the Earth is so
much more dense. The inclination of the Moon’s
orbit is also significantly different from what would
be expected had the two bodies formed together.
c. The impactor hypothesis imagines a collision
between the very young Earth and a Mars-sized
body, which derived enough material from Earth’s
mantle to form a smaller body composed of less
dense, mantle composition. The core of the
impactor may have sunk into the Earth’s core to
contribute to its greater overall density. This last
hypothesis most successfully explains
compositional differences between the Earth and the
Moon, and currently is most widely accepted.
2. Volcanic activity even today releases large volumes
of gases and water vapor, and the composition of
these gases provides clues to the composition of
Copyright © Houghton Mifflin Company. All rights reserved.
179
Earth’s first permanent atmosphere. These gases
were mainly water vapor, carbon dioxide, and
nitrogen, with lesser amounts of carbon monoxide,
hydrochloric acid, and sulfur dioxide. Methane and
ammonia would have formed by chemical reactions
in the atmosphere. Oxygen was completely absent
in this early atmosphere, as evidenced by the
existence of sedimentary pyrite in Archean rocks.
Two processes brought about significant oxygen
production in the Archean Eon. In the atmosphere,
water molecules are split by ultraviolet radiation
into free hydrogen that is lost to space, and free
oxygen, which cannot escape the Earth’s gravity
and remains. Free oxygen was produced in much
greater amounts by the process of photosynthesis, in
which plants use water and carbon dioxide to make
a simple sugar compound and free oxygen. The
oxygen produced was dissolved in the oceans and
rapidly bound to metals to precipitate as insoluble
iron oxides, forming the widespread banded iron
formations of the late Archean and early
Proterozoic. By late Proterozoic times, oxygen
levels due to rampant photosynthesis had bound
most of the dissolved iron in the oceans, and iron
oxides began to appear on land in sediments known
as redbeds. Elevated oxygen levels also formed the
ozone layer in the upper parts of the atmosphere,
which thereafter protected the Earth’s surface from
ultraviolet radiation.
3. Glacial episodes of the late Proterozoic appear to
have been the most extreme Earth has ever faced,
with tropical glaciation and possibly frozen oceans.
This “snowball” Earth may have developed because
of the reduced energy output of the early Sun, and
the cyclic imbalance between chemical weathering
that utilizes carbon dioxide, and limestone
formation that locks up carbon dioxide. The
Proterozoic atmosphere was much richer in carbon
dioxide than it is today, and this probably caused a
significant greenhouse effect on Earth, which kept it
warm despite the decreased solar energy output. As
continents rifted apart, plate margins favorable for
limestone production appeared, and levels of carbon
dioxide fell as limestone formation soared. With
dropping temperatures, the oceans froze and
limestone production ceased as glaciation became
widespread. Volcanic activity continued with
continental rifting, raising levels of carbon dioxide.
The Earth warmed and the oceans melted, causing
limestone production to begin again. In tectonic
situations of continental collisions such as the
formation of Rodinia, environments favorable for
limestone formation would be reduced and the
greenhouse effect would continue unchecked. The
fact that glacial deposits are interbedded with
shallow marine carbonates provides evidence for
this late Proterozoic climatic rollercoaster.
Copyright © Houghton Mifflin Company. All rights reserved.
CHALLENGE TEST
Completion
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Hadean; Archean; Proterozoic
planetary capture; dual accretion; impactor
ultramafic
partial melting
pillow lavas
rifting
deep-water
sandstone
continental collision
orogen
Grenville Orogeny
tillites
endosymbiosis
Multiple-Choice Questions
1.
2.
3.
4.
5.
6.
7.
8.
a
c
c
d
b
c
b
d
True or False Items
1. False Earth’s oldest crust is exposed in rocks of the
Northwest Territories in Canada.
2. False The age of Precambrian rocks is determined
by radiometric dating.
3. True
4. True
5. True
6. False Although stromatolites are the most common
Archean and early Proterozoic fossils, the Ediacaran
fauna of late Proterozoic age represents advanced
multicellular eukaryotes called metazoans. In these
organisms, groups of cells perform specialized
tasks.